The innate response to pathogen-derived products has been well-described. In general terms, exposure of macrophages or dendritic cells to bacteria or bacterial products results in a predictable array of inflammatory cytokines, produced in response to a well-described series of signaling intermediates. This has not been the case with the production of IL-10. Although it is well-appreciated that IL-10 is a regulatory cytokine that can dampen the initial inflammatory response, the mechanism(s) of IL-10 gene regulation and the conditions under which IL-10 is produced are not well understood. In the present application we seek to understand how macrophages and dendritic cells make the decision to produce IL-10. In the previous support period we made the observation that stimulating macrophages in the presence of immune complexes could induce the production of high levels of IL-10. In this renewal we propose to use this model of IL-10 superinduction to understand how the IL-10 gene is regulated. We will examine the signal transduction pathways leading to IL-10 production. We present data indicating that activation of the MARK, ERK, is required for IL-10 production. ERK activation leads to histone H3 phosphorylation at the IL-10 promoter, making the promoter more accessible to transcription factors. We think these studies on the regulation of IL-10 are important because IL-10 can prevent the dysregulated overproduction of inflammatory cytokines, which can be deleterious to a host during autoimmune disease. Conversely, the overproduction of IL-10 can suppress an otherwise productive immune response and lead to susceptibility to infections. Thus, controlling IL-10 production may represent a key to regulating host immune/inflammatory responses. The specific aims of this proposal are to study: 1. The molecular mechanisms that allow the MARK pathway to influence IL-10 production. 2. The role for NF-?B1 in IL-10 gene regulation. 3. The heterogeneity of activated macrophages. Our goal is to understand how IL-10 is regulated so we can develop molecules to enhance or inhibit its production.